1. Field of the Invention
The present invention relates generally to ion implantation of silicone rubber and, more importantly, to an ion implantation process for silicone rubber designed to change the silicone rubber's surface to one characterized by low friction, being antithrombotic, inkable, wear resistant, deformable and hydro-compatible.
2. The Prior Art
Silicone rubbers find important and varied uses in industrial and medical device applications. In the medical field, manufacturers use silicone rubber, inter alia, in forming seals and valves for many groups, for membranes for insulators in electrostimulation devices and pacemakers, for artificial heart diaphragms, in penile implants, for mammary prostheses and the like. Silicone rubber is favored in the applications because it is biocompatible, is permeable to gases, is easily molded and is highly deformable.
Silicone rubber is not without its drawbacks, however. It evinces high friction against metals, other polymers and itself. Such high friction requires the addition of lubricating agents, such as fluorosilicone oil, in many applications. When the addition of lubricating agents is not possible, as in certain in-vivo implants, it often leads to premature failure due to abrasive wear and/or cracking. This property of high friction in silicone rubbers is the result of the surface morphology and of the surface energy. During their molding, whether by companion, transfer or injection molding, silicone rubber products at times acquire a rough surface morphology due to the compressive stresses built into them from the molding operation. Secondly and more importantly, certainly in medical device applications, bodily tissue and cells have a tendency to stick and stay stuck onto the silicone rubber surface, whence they may enter the vascular system, causing occlusion, clotting and strokes Since silicone rubber is non-planar and hydrophobic due to its surface energy, it has not been easily inkable.
Attempts to improve the surface properties of silicone rubber products have, for the most part, concentrated on providing suitable coatings therefor. Such coatings have included hydrogel or fluorosilicone lubricants, sulfonation processes or the employment of special adhesives. See U.S. Pat. Nos. 4,100,309 and 4,119,094 of Michael J. Micklus et al., "Coated Substrate Having a Low Coefficient of Friction Hydrophilic Coating and a Method of Making the Same". Each of these coatings introduces, however, "shelf-life" problems to the silicone rubber products. Lubricants used in implanted drug-pumps wear off with time and enter the vascular system, being the source of potentially life-threatening complications. Hydrogel coatings also can detach from silicone rubber devices due to excessive heat and/or hydration, heralding a wealth of complications and device failures. And the sulfur groups from the applied sulfonation processes severely limit the shelf-lives of the silicone rubber devices.
More recently, a Japanese corporation, Terumo Corporation, has investigated the effects of ion implantation on plasma protein adsorption onto silicone rubber sheets. See Yoshiake Suzuki et al, "Effects of Ion Implantation on Protein Adsorption onto Silicone Rubber," MRS Symposium Proceedings (1989), vol. 110, pp. 669-679. The article notes in the Abstract, inter alia, that "Ion implantation causes the surface roughness to increase by 1-5 times," see lines 5-6 of the Abstract. The Abstract also states that "The results of XPS measurements showed that implanted elements were incorporated in a gaussian like distribution and host elements were redistributed in the polymer matrix." See Abstract, lines 10-12. Our aims are opposed to both of those observations. We are interested in reducing surface roughness to achieve low surface friction and smoothness. We also are interested in improved wear resistance and improved anti-thrombogenicity. We are not interested in improving plasma protein adsorption onto the surfaces of the ion implanted silicone rubbers, however, which is a sort of protective coating.